Transcript for US military drops a 22,000-pound bomb in eastern Afghanistan

Thursday night and we begin with the U.S. Dropping that massive bomb on Afghanistan. The Pentagon saying they were targeted ISIS. President trump said he game authorized. A conventional bomb this size has never been used in combat by the U.S. Before. It comes just one week after the missile strike on Syria and we begin tonight with ABC's chief foreign correspondent Terry Moran. Reporter: The behemoth bomb found its target in the rocky terrain of eastern Afghanistan. The U.S. Military unleashing a mighty show of force against ISIS. We targeted a system of tunnels and caves that ISIS fighters used to move around freely, making it easier for them to target U.S. Military advisers and Afghan forces. In the area. Reporter: Never before has the United States used a conventional weapon this big in combat. It's officially called, a massive ordinance air blast, but it's better known by that nickname, the mother of all bombs. 22,000 pounds, so massive it must be dropped from the cargo hold of a c-130 plane. This test video shows how it's rolled out the open door tethered to a parachute before plunging towards its target. Today, president trump applauded the strike. This was another very, very successful mission. Reporter: The president was asked if he personally authorized the bombing. Turns out, he did not. What I do is I authorize my military. We have the greatest military in the world and they've done a job, as usual. So, we have given them total authorization. And that's what they're doing. And frankly, that's why they've been so successful lately. In fact, the planning for today's action began during the Obama administration. The top U.S. General in Afghanistan moving the bomb into position before trump took office. A Pentagon official tells us it was "The right weapon, the right target, and the desired effect." Though, at this hour, we still don't know how many ISIS fighters it killed. This is the second major American military strike in a week. The first, those tomahawk missiles launched into Syria, in an attempt to hobble bashar Al Assad and deter more chemical weapons strikes. The president presided over that decision, assembling his team in a mobile war room at his mar-a-lago resort. Later recalling how he broke the news to Chinese president XI that night over dinner. We had finished dinner, we're now having dessert. And we had the most beautiful piece of chocolate cake that you have ever seen, and president XI was enjoying it. So, what happens is I said, we've just launched 59 missiles, heading to Iraq. Headed to Syria? Yes, heading toward Syria, and I want you to know that. Let's get to foreign correspondent Terry Moran. Back to this massive bomb drop in Afghanistan. The Pentagon said it took pains to avoid civilian casualties. But it's already drawing an angry response from a former Afghan leader. Hamid Karzai lashing out at this attack. He it's up to Afghans to stop the usa. David. Terry Moran with us. I want to get right to retired marine colonel Steve ganyard. Can you help put the size of this bomb in perspective. No other weapon in the U.S. Inventory short of a nuclear weapon short weapon. We know the terrain in Afghanistan is notoriously challenging. This bomb massive in scope. Not the size that might not just make it more effective. It's some of the most rugged terrain in the world. As this weapon dropped and hit the valley floor the blast would have been focused and pushed into these caves to take out of these ISIS fighters. Steve, thank you for joining us This comes one week after

This transcript has been automatically generated and may not be 100% accurate.

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